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Vi–ii–V–I

In music, the vi–ii–V–I progression is a chord progression (also called the circle progression for the circle of fifths, along which it travels). It is "undoubtedly the most common and the strongest of all harmonic progressions" and consists of "adjacent roots in ascending fourth or descending fifth relationship", with movement by ascending perfect fourth being equivalent to movement by descending perfect fifth due to inversion.

The circle progression is commonly a succession through the seven diatonic chords of a diatonic scale, by fifths, including one progression by diminished fifth, returning at the end to the starting chord (in C: between F and B) and one diminished chord (in C: B):

I-IV-vii-iii-vi-ii-V-I             

Shorter common progressions may be derived by selecting certain specific chords from the series completing a circle from the tonic through all seven diatonic chords, such as the primary triads book-ending the progression:

I-                  V-I = I-V-I     
I-IV-               V-I = I-IV-V-I  

The ii-V-I turnaround lies at the end of the circle progression, as does the vi-ii-V-I progression of root movement by descending fifths, which establishes tonality and also strengthens the key through the contrast of minor and major.

The circle progression may also contain dominant seventh chords.